In order to clean the exhaust gas discharged from an engine of a vehicle, a catalyst unit such as a three-way catalyst is used. Further the supply state of fuel and air is controlled according to the concentration of oxygen in the exhaust gas.
In order to detect the concentration of oxygen in the exhaust gas, an oxygen senor (O2 sensor), for example, a lambda sensor of the Nernst type is used. This sensor generates an electromotive force according to the concentration of oxygen.
This kind of oxygen sensor suffers from a disadvantage where an abnormality such as a shorting developed at the oxygen sensor might hinder the oxygen sensor from detecting the concentration of oxygen. Thus, U.S. Pat. No. 5,298,865 teaches a technology for detecting an abnormality of the oxygen sensor.
In this technology, as shown in FIG. 12, elements such as a pull-down resistor P4, an input resistor P5, and a capacitor P6 are connected to an output line P3 extending from an oxygen sensor P1 to an ADC (A/D converter) P2 connected to a microcomputer MC, and an offset voltage circuit P8 is connected to a ground line P7 to apply an offset voltage to the oxygen sensor P1.
As shown in FIG. 13A, in the case where the output voltage Vout of the oxygen sensor P1 is decreased below the offset voltage Voff during the operation of the oxygen sensor P1, it is determined that a low voltage-side shorting (ground shorting) is developed. In the case where the output voltage Vout is increased excessively above a usual range (1 V+Voff) of a sensor output, it is determined that a high voltage-side shorting is developed.
However, for example, in either case where a break or a low voltage-side shorting occurs in the output line P3 from which the output of the oxygen sensor P1 is taken out, the output voltage Vout of the oxygen sensor P1 is made lower than the offset voltage thereby to be brought to 0 volt. It is impossible to discriminate between the break and the low voltage-side shorting.